In the United States, it is estimated that 17-30% of the population experience symptoms compatible with Irritable Bowel Syndrome (IBS). IBS is a functional condition characterized by change in bowel patterns (e.g., constipation, diarrhea), abdominal pain and bloating. It affects more women than men, causing significant symptom distress, interfering with functional activities and quality of life. The number of potential causes for IBS, the chronicity of the problem and the lack of a specific IBS marker make the conduct and evaluation of therapeutic trials challenging. Prior work in the applicant's laboratory indicates that women (ages 20-45) with IBS are distinguishable from control women using daily reports of GI discomfort frequency/severity, self-reported psychological distress, presence of psychopathology, and indicators of physiological arousal (urine catecholamines and cortisol) under baseline conditions. The purpose of the present study is to: 1) compare women with medically diagnosed IBS and asymptomatic control women with respect to autonomic nervous system (ANS) balance, ANS function, and physiological arousal under a challenge-state and basal conditions; 2) determine the relationships among indicators of ANS balance, ANS function, physiological arousal variables, psychological distress, psychopathology, and GI symptom patterns; 3) explore and identify subgroups of women with IBS. In prior work, the investigators observed differences between subgroups of women with IBS using cluster analysis techniques. These subtypes varied on the degree of physiological arousal, frequency/severity of abdominal discomfort, and psychopathology. If such subtypes can be corroborated, differing therapeutic interventions for each subtype could be construed and ultimately tested. Women with IBS (n = 144) ages 20-45 will be recruited from a medical center clinic as well as gastroenterology and primary care clinics of a large HMO (Group Health Cooperative). Non-symptomatic women (n = 72) will be recruited from both sites by posted advertisements at both sites (UW, Group Health). Women will be followed for 1 menstrual cycle using daily symptom diaries. Ovulation will be determined using an LH ovulation kit and laboratory visits for heart rate variability and ANS function, and physiological arousal will be scheduled during the mid-luteal phase of the cycle. Without a clear understanding of the role of ANS functioning and physiological arousal in IBS, the development of cost-effective therapeutic modalities that produce symptom alleviation and reduce health care costs may not be possible.